1. Field of the Invention
This invention relates to valves, in particular to valve assemblies having automatic-lift type valve members.
2. Description of the Prior Art
Compressors and pumps are commonly provided with valve assemblies having automatic-lift type valve members. The valve assemblies generally include a fluid passage having inlet and outlet portions with a valve seat therebetween. A typical automatic-lift type valve member includes a face for engaging the valve seat and preventing fluid flow between the passage inlet and outlet portions with the valve member in a closed position. The valve member is normally biased toward its closed position by a return spring or springs and is opened when fluid pressure in the passage inlet portion is sufficiently greater than that in the passage outlet portion. Such a pressure differential may be caused by, for example, the suction and discharge strokes of a reciprocating compressor piston.
The fluid pressure differential required to open the valve member is a function to several factors; including the amount of surface area on both sides of the valve member exposed to fluid pressure in the inlet and outlet portions of the passage, the amount of cohesion or "stiction" between the valve member face and the valve seat and the amount of force exerted by the return springs. In compressors, pumps and the like, reducing the fluid pressure differential required to open the valves will generally result in an increase in operating efficiency, because less "work" is thus required to move an equivalent amount of fluid and because lower fluid temperatures will be maintained.
Increasing operating efficiency is particularly desirable in compressors which are employed to transport large quantities of natural gas over great distances through pipelines from gas fields to consumers. Since natural gas engines often power such compressors, gas which is consumed thereby reduces the amount available for transmission and increases operating costs.
Reliability is also important in compressor valve assemblies because valve failures account for a large percentage of compressor downtime. Such downtime is particularly costly in natural gas transmission compressors because of the revenue loss which results from a suspension of operation. Also, if the compressor is located in a remote pumping station, additional downtime may be incurred while the necessary service personnel and parts are brought to the site.
A variety of different types of automatic lift-type valve assemblies have heretofore been proposed in an effort to achieve efficiency and reliability. For example, poppet valve assemblies with "poppets" comprising heads and guide stems are well known. Plate valve assemblies typically include a plurality of concentric, annular fluid inlet passages selectively sealed by respective concentric, annular plate valve members. Other types of automatic-lift type valve assemblies which may be encountered in compressors, pumps and the like include reed, channel, daisy and feather valve assemblies.
In operation, the valve members in such valve assemblies open and close very rapidly, e.g. one thousand times or more per minute, and strike respective valve seats with considerable force. It has been found that the useful life of such a valve member may be prolonged by providing a substantial contact area between the valve member face and the valve seat to absorb the shock upon engagement as the valve member closes. However, increasing the contact area may be detrimental to valve efficiency because less surface area on the inlet side of the valve member is thereby exposed to the fluid passage such that a greater inlet fluid pressure is required to open the valve member. Also, a greater cohesion or "stiction" between the faces is encountered. Therefore, the necessary fluid pressure differential for operation of the valve assembly tends to increase in proportion to the contact area with a corresponding decrease in efficiency.
Prior art valve assemblies with automatic-lift type valve members have heretofore not provided effective means for alleviating the efficiency problems caused by relatively large contact areas.